1. Field of the Invention
This invention relates to a system and a method for controlling the interstand tension imparted to a workpiece being rolled by rolling stands of a tandem rolling mill.
2. Description of the Prior Art
In a rolling operation on a workpiece, such as steel sheet or plate and shape or section of steel, to be rolled by a tandem rolling mill, it is desirable that the interstand tension imparted to the workpiece being rolled by rolling stands of the tandem rolling mill be maintained at a predetermined constant value. This is especially important for eliminating dimensional errors due to variations in the interstand tension, that is, deviations of the thickness and width of the workpiece from the predetermined values. In a tandem rolling mill designed for producing section or shape steel bar or the like, the above requirement is also important for eliminating dimensional errors and non-uniformity of the section of the products.
Some of the inventors of the present invention have proposed a method and a system for controlling the interstand tension in a tandem rolling mill without the use of a mechanical looper. Such a method and system are disclosed in, for example, U.S. Pat. Nos. 3,940,960 and 4,137,742. In these U.S. patents, the interstand tension is indirectly detected or arithmetically computed on the basis of physical quantities relating to the tension imparted to a workpiece being rolled and is then compared with a reference or desired value, and the rolling speed of the rolls is controlled to cancel the difference or error therebetween, whereby the interstand tension can be controlled to be maintained constant throughout the rolling operation. In other words, the rolling force P and the rolling torque G are detected to indirectly detect the interstand tension so that the interstand tension can be maintained at the desired value throughout the rolling operation.
The method and system disclosed in these U.S. patents are basically satisfactory in that the interstand tension control in a tandem rolling mill can be generally effected with good accuracy.
However, it has been found during practical rolling operation by a tandem rolling mill incorporating the system of the above noted U.S. patents, a problem arises in that the interstand tension control system acts, in response to an excessively large interstand tension variation, indirectly detected by the system in the final stage of rolling operation, on each of individual workpieces to compensate for the tension variation, sometimes resulting in hunting which prevents stability of the control.